This invention relates to an apparatus and method for the detection of simultaneous call transmissions, in particular to double-sideband amplitude modulation (DSB-AM) transmissions.
Simultaneous Call Transmissiofn (SCT) is a situation when two or more transmissions occur simultaneously on the same frequency band. The end listener is then (usually) only able to understand the higher powered of the pair of transmissions. An example is illustrated in FIG. 1.
This is a potentially hazardous situation as the sender of the weaker transmission may assume that they were actually heard by the end listener and take action accordingly. A situation where the consequent action would be incredibly dangerous would be where two planes transmit a signal to an air-traffic controller simultaneously, who then replies, and both planes believe the response is directed to them. Such a scenario may be noticed by an alert, trained human operator listening out for the characteristic phenomena of interfering voice and heterodyne tones arising from the frequency difference between the two transmitters. However this only reliably works when the weaker transmitter is (1) within the power range of the stronger transmitter e.g. 0 to −20 dB and (2) the heterodyne lies in the filtered audio range (e.g. 300 Hz to 3.5 kHz). The first condition may not be met if one transmission originates much further away than the other (i.e. one plane is overhead and the other is several kilometres away); the second condition may not be met if the frequency difference between the two transmitters (defined by the precision of the quartz in the transmitting equipment) is minor.
Indeed, SCT can occur far outside of these values due to real world effects such as propagation loss, multipath error and frequency error.
Hence automatic SCT detection is a desirable feature for radios.